Wireless Local Area
Networks (WLANs)
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WLAN Technologies
• IEEE 802.11
• Hiperlan
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IEEE 802.11
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Bibliography
1. Tutorial on 802.11 WLAN 802.11,by Crow, in the
IEEE Communications Magazine, 1997 (in
English)
2. A.Tanenbaum, Computer networks, 4th ed.,
Prentice-Hall, 2002 (in English)
3. M. S. Gast, 802-11 Wireless Networks - The
definitive Guide, O’Reilly 2002 (in English)
4. Supporting Material (in English)
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IEEE 802.11
• Wireless LAN standard specifying a wireless
interface between a client and a base station (or
access point), as well as between wireless clients
• Defines the PHY and MAC layer (LLC layer defined
in 802.2)
• Physical Media: radio or diffused infrared
• Standardization process begun in 1990 and is still
going on (1st release ’97, 2nd release ’99, ‘03)
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Protocol Stack
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Standards Evolution
•
•
•
•
•
•
•
•
•
•
•
•
•
•
IEEE 802.11 - The original 1 Mbit/s and 2 Mbit/s, 2.4 GHz RF and IR standard
(1999)
IEEE 802.11a - 54 Mbit/s, 5 GHz standard (1999, shipping products in 2001)
IEEE 802.11b - Enhancements to 802.11 to support 5.5 and 11 Mbit/s (1999)
IEEE 802.11c - Bridge operation procedures; included in the IEEE 802.1D standard
(2001)
IEEE 802.11d - International (country-to-country) roaming extensions (2001)
IEEE 802.11e - Enhancements: QoS, including packet bursting (2005)
IEEE 802.11f - Inter-Access Point Protocol (2003) Withdrawn February 2006
IEEE 802.11g - 54 Mbit/s, 2.4 GHz standard (backwards compatible with b) (2003)
IEEE 802.11h - Spectrum Managed 802.11a (5 GHz) for European compatibility
(2004)
IEEE 802.11i - Enhanced security (2004)
IEEE 802.11j – Spectrum extensions for Japan (2004)
IEEE 802.11n – High-speed (up to 540 Mb/s) WLAN
IEEE 802.11p - WAVE - Wireless Access for the Vehicular Environment
IEEE 802.11s - ESS Mesh Networking
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IEEE 802.11 PHY Layer Activities
PHY Layer
IR
802.11 1-2Mbps
2.4GHz FHSS
802.11 1-2Mbps
2.4GHz DSSS
802.11 1-2Mbps
5GHz OFDM
802.11b 5-11Mbps
802.11d / TG d
Regulatory Domain
Update
802.11a 6-54Mbps
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802.11g
2.4GHz OFDM
54Mbps
(approved in
June’03)
802.11h 5GHz
Spectrum
Managment
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IEEE 802.11 MAC Layer Activities
MAC Layer
802.11e / TG e MAC
Enhanced QoS
802.11 MAC
802.11f / TG f
Inter-AP Protocol
High
Throughput
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Radio
Resource
Managment
802.11i / TG i
Security Mechanisms
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IEEE 802.11 (Radio) Evolution
802.11
802.11b
(Wi-Fi)
802.11a
802.11g
Approval
July 1997
Sep. 1999
Sep. 1999
June ‘03
Bandwidth
83.5 MHz
83.5 MHz
300 MHz
83.5 MHz
Operation
frequency
2.4-2.4835
GHz
2.4-2.4835
GHz
5.15-5.35 GHz
5.725-5.825 GHz
2.4-2.4835
GHz
No. of nonoverlapping
channels
3 Indoor /
Outdoor
3 Indoor /
Outdoor
4 Indoor
4 Indoor/Outdoor
3 Indoor /
Outdoor
Standard
Data rate /
channel
1,2 Mbps
1,2,5.5,11
Mbps
6,9,12,18,24,36,
48,54 Mbps
1,2,5.5,6,9,
11,12,18,24,
36,48,54Mb
ps
PHY layer
FHSS, DSSS
DSSS
OFDM
DSSS /
OFDM
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802.11 Architecture
• BSS (Basic Service Set): set of nodes using the same
coordination function to access the channel
• BSA (Basic Service Area): spatial area covered by a
BSS (WLAN cell)
• BSS configuration mode
– with infrastructure: the BSS is connected to a fixed
infrastructure through a centralized controller, the
so-called Access Point (AP)
– ad hoc mode
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WLAN with Infrastructure
• BSS contains:
– wireless hosts
– access point (AP): base
station
• BSS’s interconnected by
distribution system (DS)
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Ad Hoc WLANs
• Ad hoc network: IEEE 802.11 stations can
dynamically form a network without AP and
communicate directly with each other
• Applications:
– “laptop” meeting in conference room, car
– interconnection of “personal” devices
– battlefield
• IETF MANET
(Mobile Ad hoc Networks)
working group
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Extended Service Set (ESS)
• Several BSSs interconnected with each other at the
MAC layer
• The backbone interconnecting the BSS APs
(Distribution System) can be a:
– LAN (802.3 Ethernet/802.4 token bus/802.5 token ring)
– wired MAN
– IEEE 802.11 WLAN
• An ESS can give access to the fixed Internet network
through a gateway node
• If fixed network is a IEEE 802.X, the gateway works as a
bridge thus performing the frame format conversion
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Possible Scenarios (1)
Ad hoc networking
Independent BSS (IBSS)
STA
STA
AP
STA
Distribution
system
STA
STA
IEEE
802.X
AP
Network with infrastructure
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STA
STA
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Possible Scenarios (2)
Ad hoc WLAN
STA
Distribution
System
STA
AP
STA
AP
STA
STA
STA
WLANs with infrastructure
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Joining a BSS
Scanning
Authentication
Association
• BSS with AP: Both authentication and
association are necessary for joining a BSS
• Independent BSS: No authentication neither
association procedures are required for joining
an IBSS
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Joining BSS with AP: Scanning
A station willing to join a BSS must get in contact with
the AP. This can happen through:
1.
Passive scanning
•
2.
The station scans the channels for a Beacon frame
(with sync. info) that is periodically sent by the AP
Active scanning (the station tries to find an AP)
•
The station sends a ProbeRequest frame
•
All APs within reach reply with a ProbeResponse
frame
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Joining BSS with AP: Authentication
Once an AP is found/selected, a station goes through
authentication
• Open system authentication (default, 2-step process)
•
Station sends authentication frame with its identity
•
AP sends frame as an ack / nack
• Shared key authentication
•
Stations and AP own shared secret key previously exchanged
through secure channel independent of 802.11 (e.g. set in AP
and typed by station user)
•
Stations authenticate through secret key (requires encryption
via WEP): challenge & response
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Joining BSS with AP: Association
• Once a station is authenticated, it starts the
association process, i.e., information exchange about
the AP/station capabilities and roaming
 STA -> AP: AssociateRequest frame
 AP -> STA: AssociationResponse frame
 New AP informs old AP via DS in case of roaming
• Only after the association is completed, a station can
transmit and receive data frames
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IEEE 802.11 / 802.11b
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Physical Layer
Three different access techniques:
• Infrared (IR)
• Frequency hopping spread spectrum (FHSS)
• Direct sequence spread spectrum (DSSS)
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Infrared
• Works in the regular IR LED range, i.e., 850-950 nm
• Used indoor only
• Employes diffusive transmissions, nodes can receive
both scattered and line-of-sight signals
• 2 Mbps obtained through 4-pulse position modulation
(4-PPM), i.e., 2 information bits encoded with 4 bits
• Max output power: 2W
• Not really used – IrDA is more common and cheaper
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Spread Spectrum
• Idea: spread signal over wider frequency band
than required
• Frequency Hopping : transmit over random
sequence of frequencies
• Direct Sequence
 random sequence (known to both sender and
receiver), called chipping code
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FHSS
• Not really used anymore
• Frequency band: ISM @ 2.4 GHz
• In the U.S., the FCC has specified 79 ISM
frequency channels with width equal to 1 MHz.
Central frequency is @ 2.402 GHz
• 3 channels each corresponding to 1Mbps with
GFSK modulation
• 20 ms dwell time  50 hops/s
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DSSS (1)
• Radiated power is limited
 Typical values: 85 mW
• Frequency band: ISM bands @ 2.4 GHz
• Band divided into 14 channels, each 22 MHz wide
• To avoid interference, only channels 1,6,11 are used
(which are spaced by  25MHz)
• No more than 3 adjacent BSSs should be allowed
• Adjacent BSSs coexist without interfering with each other if
the separation between their f0 is at least equal to 25MHz
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Overlapping Frequency Channels
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Channel 1
Channel 6
Channel 11
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Advantage of Multi-rate
• Direct relationship between
1 Mbps
communication rate and the
2 Mbps
channel quality required for that
5.5 Mbps
rate
11 Mbps
• As distance increases, channel
quality decreases
• Thus tradeoff between
communication range and link
speed
• Multi-rate provides flexibility to
meet both consumer demands
Lucent Orinoco 802.11b card ranges using and coverage requirements
NS2 two-ray ground propagation model
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Rate Adaptation
• Stations constantly perform operations to detect
and automatically set the best data rate
• Control information always sent @ basic rate
• Standard does not specify how to adapt
transmission speed
• Automatic Rate Adaptation: based on SIR
measurements over moving window
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Auto Rate Selection
• Auto Rate Fallback (ARF) [Monteban97]
– Adaptive, based on success/failure of previous packets
– Simple to implement
– Doesn’t require the use of RTS/CTS or changes to 802.11
specs
• Receiver Based Auto Rate (RBAR) [Holland01]
– Receiver uses SNR measurement of RTS to select rate and
notifies it to the sender through CTS
– Faster & more accurate in changing channel
– Requires some tweaks to the header fields
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IEEE 802.11 MAC Protocol
Performs the following functions:
 Resource allocation
 Data segmentation and reassemby
 MAC Protocol Data Unit (MPDU) address
 MPDU (frame) format
 Error control
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Time Units (Slots)
• Time is divided into intervals, called slots
• A slot is the system unit time and its duration depends
on the implementation of the physical layer (it accounts
for TX/RX turnaround time and Power detection time)
• 802.11b: 5 μs turnaround + 15 μs power detection = 20 μs
• Stations are synchronized with the AP in the
infrastructure mode and among each other in the ad
hoc mode  the system is synchronous
• Synchronization maintained through Beacon frames
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IFS – InterFrame Space
• InterFrame Space (IFS)
• time interval between frame transmissions
• used to establish priority in accessing the channel
• 4 types of IFS:
 Short IFS (SIFS)
 Point coordination IFS (PIFS) >SIFS
 Distributed IFS (DIFS) >PIFS
 Extended IFS (EIFS) > DIFS
• Duration depends on physical level implementation
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Short IFS (SIFS)
• To separate transmissions belonging to the
same dialogue
• Shortest IFS  Associated to the highest priority
• Its duration depends on:
 Propagation time over the channel
 Time to convey the information from the PHY to
the MAC layer
 Radio switch time from TX to RX mode
• 802.11b: 10μs
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Point Coordination IFS (PIFS)
• Used to give priority access to Point Coordinator
(PC)
• Only a PC can access the channel between SIFS
and DIFS
• PIFS=SIFS + 1 time slot
• SIFS < PIFS
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Distributed IFS (DIFS)
• Used by stations waiting for a free channel to
contend
• Set to: PIFS + 1 time slot
• SIFS < PIFS < DIFS
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Extended IFS (EIFS)
• Used by a station when the PHY layer notifies
the MAC layer that a transmission has not been
correctly received
• Waits more before trying to access the channel,
as a different station may correctly receive the
frame and reply with an ACK, and we do not
want to disrupt the ACK with a new transmission
• SIFS < PIFS < DIFS < EIFS
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MAC Frames
Three frame types are defined
1. Control: positive ACK, handshaking for
accessing the channel (RTS, CTS)
2. Data Transfer: information to be transmitted
over the channel
3. Management: connection establishment/release,
synchronization, authentication. Exchanged as
data frames but are not reported to the higher
layer
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Data Transfer
• Distributed, asynchronous data transfer for delaytolerant traffic (like file transfer)
 DCF (Distributed Coordination Function)
• Centralized, synchronous data transfer for real-time
traffic (like audio and video)
 PCF (Point Coordination Function): based on the
polling of the stations and controlled by the AP (PC)
 Its implementation is optional (not really
implemented)
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DCF Access Scheme
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DCF basic features
• DCF implementation is mandatory
• Broadcast wireless medium: multiple access
• Distributed scheme: lack of central coordination
Random Multiple Access
• Stations have a single network interface, and
can perform only one action at a time: trasmit
or receive (no Collision Detection)
CSMA/CA
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CSMA
• Carrier Sense Multiple Access
• If a node needs to transmit data
– senses the channel (Carrier Sensing) for a DIFS period
– if the channel is idle after DIFS, the station transmits
– if the channel becomes busy during the DIFS period, the
station waits until the transmission is ended before trying
to transmit again
• If a node receives data correctly
DIFS
DATA
SIFS
DIFS
– replies with an ACK after SIFS from end of data reception
ACK
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destination
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CSMA
• Carrier Sensing is performed in two ways in DCF
• Physical Carrier Sensing: the station senses the channel by
means of its network interface
• Virtual Carrier Sensing: the station uses information about
ongoing data transmissions to avoid transmission
DATA
DIFS
SIFS
DIFS
– when DATA is received, other stations set a Network Allocation
Vector (NAV) to the end of data exchange (ACK included), and stay
silent until the NAV expires
ACK
NAV
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destination
other station
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CSMA
DIFS
• Random multiple access: stations contend for the
channel
• Each transmission requires a contention
one
single data frame sent every time
• Collisions can occur
collision
DATA
source A
DATA
source B
• Wireless channel can cause errors on bits
• Automatic Retransmission reQuest (ARQ)
stop&wait used to retransmit non-ACK’d frames up
to retryLimit times
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CSMA/CA
• CSMA with Collision Avoidance
DATA
source A
destination
source B
4 3 2 1
SIFS
BO 4
DATA
ACK
BO 8
8 7 6 5
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source A
NAV
destination
with BO
DIFS
DATA
without BO
DIFS
– When a station senses the channel idle after DIFS it
starts a Random BackOff (BO) before transmitting data
source B
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CSMA/CA
• After DIFS expires (and channel is still idle!)
–
–
–
–
contending stations each extract a BO
BOs are decremented by each station
the first station whose BO goes to zero transmit
other stations
• sense a transmission has started
• freeze their BO to the current value
• set their NAV to the end of the transmission
• When transmission ends (after the ACK)
– contending stations all wait DIFS
– contending stations resume their BOs decrement
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CSMA/CA
• A station successfully completing a transmission,
always extracts a new BO (Post-BackOff), even if it
has no data waiting to be sent
• After DIFS from ACK reception, it starts
decrementing the Post-BackOff, which behaves like
a standard one
• This means that a station can wait just DIFS before
sending data in two cases only (assuming an idle
channel)
– the station has just joined the BSS
– the station receives a packet to send after it has already
decremented its Post-BackOff to zero
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CSMA/CA
2 1
8 7 6 5
4 3 2
ACK
6 5
received
packet to
send
NAV
1
backoff
A
BO 6
2 1
DATA
D
1
B
SIFS
3 2
6 5 4 3
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DIFS
BO 3
SIFS
DIFS
SIFS
4 3 2 1
SIFS
DIFS
• Example: 3 contending stations
BO 4
new BackOff
extraction
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C
CSMA/CA
• Collisions are still possible
– two stations can extract the same BO value
• The probability of a collision depends on the
number of contending stations
– more contending stations
higher probability that two
stations pick the same BO value
• To reduce the probability of collision in presence of
many stations, the range of the BO is increased
– more BO values to pick from
lower probability that
two stations choose the same BO value
– disadvantage: delay is increased
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CSMA/CA
• Random Backoff is computed as
BO = slotTime * uniform[0,CW]
• CW is the Contention Window:
– CW is an integer always in the interval [CWmin,CWmax]
– CW is initially set to CWmin
– CW is doubled after every failed tranmsission, up to a
value CWmax
CW = 2 ( CW + 1 ) - 1
– CW is reset to CWmin after a successful transmission
– Standard values for 802.11 DCF:
CWmin
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31, 63, 127, 255, 511, 1023
CWmax
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CSMA/CA
• The PHY layer can inform the MAC layer that an
erroneous transmission has been sensed
• The error might be related to the position of the
station, and other stations might receive the frame
correctly
• As a consequence, the station that sensed the
erroneous frame (A) must stay silent for the time
needed for a possible reply (ACK) from the
destination of the transmission
• Station A waits EIFS after the end of reception of
the erroneous frame (when channel becomes idle)
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DATA
SIFS
DIFS
4 3 2 1
source +
destination
ACK
DATA
ACK
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DATA
SIFS
4 3 2 1
EIFS
DIFS
BO 4
other station
source +
destination
other station
ACK
source +
destination
ACK
other station
with EIFS
EIFS
4 3 2 1
DIFS
BO 4
SIFS DIFS
DIFS
1 DATA
without EIFS
BO 4
CSMA/CA
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CSMA/CA: problems
• Long time to detect a collision
– must wait for missing ACK, the whole frame must be
transmitted
• Hidden Terminal
– stations may not be all within transmission range
SIFS
DIFS
OR
4 3 2 1
2 1
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DATA
DATA
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RTS/CTS
• Solution: handshaking phase before data
transmission
CTS
DATA
SIFS
SIFS
2 1 RTS
SIFS
DIFS
SIFS
– the sender asks permission to transmit with a Ready To
Send (RTS) control frame
– The receiver grants transmission with a Clear To Send
(CTS) control frame
– All the handshaking control frames are sent at basic
transmission rate (usually 1Mbps) to ensure maximum
resilience to channel errors
ACK
NAV
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RTS/CTS
• Neighboring stations all set/update their NAV upon
every RTS/CTS/DATA/ACK frames reception
• RTS (20 bytes) and CTS (14 bytes)
– small frames
– still add overhead to 802.11 transmission
• RTS/CTS handshaking only used for large frames
– only packets larger than a RTS/CTS threshold are
preceded by a RTS/CTS handshaking
– The RTS/CTS threshold also determines the maximum
number of retransmissions of a packet
• shortRetryLimit (7) if packet size ≤ RTS/CTS threshold
• longRetryLimit (4) if packet size > RTS/CTS threshold
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RTS/CTS
• Long collision detection times are avoided
2 1 RTS
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ACK timeout
2 1 RTS
source B
BO 4
8 7 6 5 4 3
CTS
DATA
SIFS
BO 2
destination
SIFS
2 1 RTS
DATA
source A
SIFS
DIFS
SIFS
2 1
DATA
DIFS
2 1
ACK timeout
DIFS
SIFS
– Collision detected SIFS after RTS transmission
ACK
NAV
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RTS/CTS
• Hidden terminal problem is mitigated
4 3 2 1
DATA
SIFS
4 3 2 1 RTS
CTS
8 7 6 5 4 3 2
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DATA
SIFS
SIFS
DIFS
8 7 6 5 4 3 2 1
SIFS
SIFS
DIFS
– intermediate station informs out-of-range nodes
of the ongoing transmission
DATA
ACK
NAV
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RTS/CTS
• RTS/CTS does not solve all the problems
• Complex Hidden Terminal
1.
sends DATA to
+
sends RTS to
2.
sends DATA to
+
sends CTS to
3.
does not receive CTS from
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(collision with

) and can disrupt

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RTS/CTS
• RTS/CTS does not solve all the problems
• Exposed Terminal
1.
sends RTS to
2.
sends CTS to
3.
receives RTS from
4. but a transmission from
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and avoids transmission to
to
would be ok!
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Data Fragmentation (1)
• An MSDU is fragmented into more than one frame
(MPDU) when its size is larger than a certain
fragmentation threshold
 In the case of failure, less bandwidth is wasted
• All MPDUs have same size except for the last MPDU
that may be smaller than the fragmentation threshold
• PHY and MAC headers are inserted in every
fragment -> convenient if the fragmentation threshold
is not too little
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Data Fragmentation (2)
• MPDUs originated from the same MSDU are
transmitted at distance of SIFS + ACK + SIFS
• The transmitter releases the channel when
 the transmission of all MPDUs belonging to an
MSDU is completed
 the ACK associated to an MPDU is lost
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Data Fragmentation (3)
• Backoff counter is increased for each fragment
retransmission belonging to the same frame
• The receiver reassembles the MPDUs into the
original MSDU that is then passed to the higher
layers
• Broadcast and multicast data units are never
fragmented
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PCF
Centralized access scheme
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Basic Characteristics
• Used for services with QoS requirements, it
provides a contention-free access to the channel
• Needs a Point Coordination (PC) that polls the
stations → it can be implemented in networks with
infrastructure only (AP=PC)
• Stations enabled to operate under the PCF mode
are said to be CF-aware (CF=Contention Free)
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PCF
• Stations declare their participation in the CF
phase in the Association Request
• PC builds the polling list based on the received
requests
• Polling list is static
• Implementation of the polling list and tables are
left to the system operator
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PCF Duration
• Designed to coexist with the DCF
• The Collision Free Period (CFP) Repetiton Interval
(or Superframe) determines the repetition frequency
of the PCF with respect to the Collision Period (CP),
during which the DCF is performed
• CFP starts with a beacon signal
• periodically broadcast by the AP
• used to synchronize stations
• The CFP terminates with a frame of CF_end
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Coexistence between DCF and PCF
CFP Repetition Interval or
Superframe
B
PCF
NAV
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DCF
B
PCF
DCF
NAV
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PCF Duration
• Max CFP duration determined by parameter
CFP_Max_Duration (included in the beacon)
 Min CFP_Max_Duration: 2 MPDUs with max
length + 1 beacon frame + 1 CFP_end frame
 Max CFP_Max_Duration: CFP repetition interval –
(RTS+CTS+1 MPDU with max length + ACK)
• CFP duration determined by PC based on traffic load
• When a CFP starts, all stations set their NAV to
CFP_Max_Duration
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Superframe and PCF Protocol
TBTT
Superframe
Max Contention Free Period
Busy
Medium
P
S
D1+
Poll
B
S
S
S
D2+AC
K+Poll
D3+
Poll
U1+
ACK
Ack
CFEnd
Contention
Period
U3+
ACK
Null+ACK
S
S
S
S
NAV
• TBTT: Target Beacon Transmission Time
• D1, D2, D3: frames sent by PC
• U1, U2, U3: frames sent by polled station
• B: beacon frame (sent by AP)
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Reset NAV
D=CF-Downlink
U=CF-UPlink
S=SIFS
P=PIFS
Copyright Gruppo Reti – Politecnico di Torino
CFP Access
• When CFP has to start, the PC senses the channel.
If idle and still so for a PIFS, the PC broadcasts the
beacon frame
• In CFP, stations can transmit only in response to a
PC’s poll, or to acknowledge an MPDU
• After SIFS from the beacon, the PC transmits
 a CF-Poll frame or
 a data frame or
 a data frame + a CF-Poll frame
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CFP Access
• The PC MAY end the CFP by sending a CFP_end
frame even right after its first transmission (a CF-ACK
or a data frame or a data+CF-ACK)
• In the case the CFP goes on, the polled station can
reply after a SIFS interval by sending
 a data frame
 a data frame + CF-ACK (if it received data)
 a NULL frame (+ ACK) if it does not have any data
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CFP Access
• As the PC receives a data frame+CF-ACK
• it waits SIFS
• then it transmits a data frame+CF-ACK+CFPoll to a different station
• If the PC does not receive the CF-ACK as
expected, it waits a PIFS time and then transmits
to the next station in the polling list
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What’s the Problem in WLAN QoS
•
PCF designed to provide QoS to real-time traffic
•
What makes QoS in 802.11 difficult?
1. Unpredictable beacon delay
 An STA does not initiate a transmission after TBTT, but
continues its on-going transmission thus beacon
frames may be delayed
 The larger the frame size, the longer the beacon delay
(up to 4.9 ms)
2. Unknown transmission duration
3. Static polling list -> polling overhead
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More details on 802.11
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Power Saving
• Typically, 802.11 cards have high power
consumption:
– Ptx=1.6 W, Prx=1.45 W, Pidle=1.15 W,
Pdoze=0.085 W
• To reduce energy expenditure, stations
can go into Power Saving Mode (PSM)
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Power Saving Mode (with AP)
• AP periodically transmits Beacon (for sync.)
• Stations which want to move into PSM select their
“waking up period” (as a multiple of the Beacon
period) and inform the AP
• The AP maintains a record of the stations in PSM
and buffers packets until stations wake up
• Upon sending a beacon, the AP includes in the
Traffic Indication Map (TIM) field which stations in
PSM have waiting data
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Power Saving Mode (with AP)
• Stations in PSM monitor beacon transmissions every
waking up period:
– if there are data for them they remain awake and poll the
AP for it
– otherwise they go back to sleep
• Multicast messages are transmitted at an a-priori
known time
• All stations who wish to receive this information
should wake up
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Power Saving Mode (with AP)
• Stations with waiting data backoff before sending a
PS-Poll message
• If PS-Poll is successful, AP sends data frame after
SIFS
• If there are more frames at the AP for that station,
AP sets the MoreData bit to 1 and the station will
send another Poll
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Generic Frame Format
(for all frames)
Preamble
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MAC
PLCP
Header Header
Frame Body
(payload)
CRC
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Preamble and PLCP Header
• Preamble (PHY dependent, @ basic rate)
– Sync - An 80 bit sequence of alternating 0s and 1s
– Start Frame Delimiter (SFD) - 16-bit pattern: 0000
1100 1011 1101 (for frame timing)
• PLCP Header (@ basic rate)
– Length Word - No. of bytes in the frame (used by
the PHY layer)
– Signaling Field – for data speed
– HEC – 16-bit CRC for the header
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MAC Header+Frame Body+CRC
2
2
6
Frame Duration Addr.
Control or ID
1
2
2
4
1
6
6
Addr.
2
Addr.
3
1
2
1
6
Sequ. Addr.
Control
4
1
1
02312
Frame
Body
1
1
4
CRC
1
Protocol
From More
Power More
WEP Order
Type
SubT
To
DS
Retry
DS Frag.
Version
Mngmt Data
Length of the MAC Data and CRC fields in octects
Length of the Frame Controld fields in bits
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Frame Control Field
• Protocol Version
– To differentiate among e.g 802.11, 802.11a, b, g
• Type and Subtype
– Frame type: management (e.g., Beacon, Probe,
Association), control (e.g., RTS, CTS, ACK, Poll),
or data
– There are more than 30 different subtypes of
frame
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Frame Control Field
• ToDS / FromDS
– Whether a frame destined to the DS or not
• FromDS=0,ToDS=0: Mng&Control frames, Data frames
within an IBSS
• FromDS=1,ToDS=0: data frame to a station in an
infrastructure network
• FromDS=0,ToDS=1: data frame from a station in an
infrastructure network
• FromDS=1,ToDS=1: data frame on a wireless bridge
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Frame Control Field
• More Fragments
– To signal more incoming fragments
• Retry
– 1 if it is a retransmission
• Power Managment
– To signal that the station is changing from Active
to Power Save mode (or vice-versa)
• More Data
– There are more frames buffered for this station
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Frame Control Field
• WEP
– Indicates whether the frame body is encrypted
or not
• Order
– The frame is in a stream that is strictly ordered
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Other MAC Header Fields
• Duration / ID
– Duration: used for NAV calculation
– ID: Station ID for polling in PSM
• Sequence Control
– Frame numbering and fragment numbering
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Other MAC Header Fields
Standard 48-bit long IEEE address
• Address 1
– Recipient address
– if ToDS=0, then end station’s address
– if ToDS=1, BSSID (if FromDS=0) or bridge (if FromDS=1)
• Address 2
– Transmitter address
– if FromDS=0, then source station’s address
– If FromDS=1, BSSID (if ToDS=0) or bridge (if ToDS=1)
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Other MAC Header Fields
• Address 3
– If FromDS=ToDS=0, BSSID
– If FromDS=0, ToDS=1, final destination address
– If FromDS=1, ToDS=1, final destination address
• Address 4
– Original source address
– Set only when a frame is transmitted from one AP to
another, i.e., if FromDS=ToDS=1
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Example: RTS Frame
Frame
Control
Duration
RA
TA
CRC
MAC Header
• Duration (in s): Time required to transmit next (data) frame
+ CTS + ACK + 3 SIFS
• RA: Address of the intended immediate recipient
• TA: Address of the station transmitting this frame
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Example: CTS Frame
Frame
Control
Duration
CRC
RA
MAC Header
• Duration (in s): Duration value of previous RTS frame  1
CTS time  1 SIFS
• RA: The TA field in the RTS frame
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Example: ACK Frame
Frame
Control
Duration
RA
CRC
MAC Header
• Duration: set to 0 if More Fragments bit was 0, otherwise
equal to the duration in previous frame  1 ACK  1 SIFS
• RA: copied from the Address 2 field of previous frame
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Some Numerical Values…
• PHY preamble: 18 bytes (long) or 9 bytes (short),
transmitted @ 1 Mbps
• PHYHDR: 6 bytes, transmitted @ 1 Mbps
• MACHDR: 34 bytes, transmitted @ same rate as the
one used to send the frame
• ACK=Preamble + PHYHDR+14 bytes
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IEEE 802.11 Evolution
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IEEE 802.11 (Radio) Evolution
802.11
802.11b
(Wi-Fi)
802.11a
802.11g
Approval
July 1997
Sep. 1999
Sep. 1999
June ‘03
Bandwidth
83.5 MHz
83.5 MHz
300 MHz
83.5 MHz
Operation
frequency
2.4-2.4835
GHz
2.4-2.4835
GHz
5.15-5.35 GHz
5.725-5.825 GHz
2.4-2.4835
GHz
No. of nonoverlapping
channels
3 Indoor /
Outdoor
3 Indoor /
Outdoor
4 Indoor
4 Indoor/Outdoor
3 Indoor /
Outdoor
Data rate /
channel
1,2 Mbps
1,2,5.5,11
Mbps
6,9,12,18,24,36,
48,54 Mbps
1,2,5.5,6,9,
11,12,18,24,3
6,48,54Mbps
PHY layer
FHSS, DSSS
DSSS
OFDM
DSSS /
OFDM
Standard
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IEEE 802.11a
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Physical Layer
• Standard approved years ago, but difficulties due
to higher frequency (5GHz) and costs
• UNII 5 GHz bands
• In U.S.:
• UNII-1: 4 channels for indoor use
• UNII-2: 4 channels for indoor/outdoor use
• UNII-3: 4 channels for outdoor bridging
• In Europe difficulties due to Hiperlan II, but now
it is approved
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Physical Layer
• OFDM (Orthogonal Frequency Division
Modulation) as transmission technology
• Very good performance against multipath
• Modulation: BPSK, QPSK, 16-QAM, 64-QAM
• Data rates: 6, 9, 12, 18, 24, 36, 48, 54 Mbps
• Reduced range
• slot=9μs, SIFS=16μs, PIFS=25μs, DIFS=34μs,
CWmin=15, CWmax=1023
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OFDM
• Orthogonal Frequency
Division Multiplexing
(OFDM) distributes data
over multiple, adjacent,
frequency channels
• Channels are narrow-band
with carriers very close to
each other
• Each channel is orthogonal
w.r.t. the others (spectra
have zeros in
correspondence of the
other carriers) -> no cochannel interference
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OFDM
• In practice, each user transmits over multiple
narrow-band channels in parallel, hence at low bit
rate
• Low bit rate transmissions imply increased
robustness against delay spread on the multipath
channel
• Continuous transmissions at low bit rate require low
power consumption
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OFDM in 802.11a
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802.11a
• Transmission speed up to 54 Mbps
• Products on the market are capable of 108 Mbps
(Atheros turbo mode)
– Will IEEE adopt this?
• Does the higher frequency have an essential
impact on the communication range?
• Corresponding to the ETSI Hiperlan II
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802.11a vs. 802.11b
• 8 independent channels with 802.11a (3 in
802.11b)
• Max data speed is 5 to 10 times higher
• Power consumption is similar, although with
802.11a it takes 4 to 9 times less energy to
transmit a given length packet (due to the higher
speed)
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802.11a vs. 802.11b
• No other existing equipment interfere (yet)
including microwaves, 802.11b or Bluetooth
• Atheros claims that during real throughput
measurements 802.11b never superseded
802.11a (in a typical office environment despite
the higher frequency band usage – see diagrams
in the next slides)
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802.11a vs. 802.11b
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Copyright Gruppo Reti – Politecnico di Torino
802.11a vs. 802.11b
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Copyright Gruppo Reti – Politecnico di Torino
802.11a vs. 802.11b
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Copyright Gruppo Reti – Politecnico di Torino
802.11a vs. 802.11b
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Copyright Gruppo Reti – Politecnico di Torino
802.11a vs. 802.11b
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IEEE 802.11g
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IEEE 802.11g
• Standard 802.11g approved in June 2003
• Operates in the ISM 2.4 GHz bands
• Backward compatible with 802.11b
• Uses OFDM as transmission technology
• Modulation: BPSK, QPSK, 16-QAM, 64-QAM
• Data rates: 1, 2, 5.5, 6, 9, 11, 12, 18, 24, 36,48, 54
Mbps
• Power consumption similar to 802.11b
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“All g” Operational Mode
• Slot time=20 μs / Short slot time=9 μs
• SIFS=10 μs, CWmin=15, CWmax=1023
• Basic rates determined by the AP (may be
greater than 1Mbps), for management and
control frames, as well as multicast and
broadcast data frames
• Actual throughput: ≈20 Mbps
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Backward Compatible
• Slot time=20μs
• SIFS=10 μs, CWmin=31, CWmax=1023
• NAV distribution
– Protection mechanisms
• CTS-to-itself
@basic rate, to notify duration to all
• RTS / CTS
same scope, better for hidden terminals
– DSSS-OFDM: frame with DSSS preamble and
header, and OFDM payload (no need for protection)
• Actual throughput: ≈10 Mbps
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Available Products
• 802.11 a/b/g combo-card
• Ad hoc mode support
• Typically power control
• Improved security functions
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HIPERLAN
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General Characteristics
• Standard ETSI (European Telecommunications
Standards Institute) HIPERLAN/1 (H/1) and
HIPERLAN/2 (H/2 (1999)
• Frequency bands: 5.15-5.30 GHz & 17.1-17.3 GHz
• H/1 bit rates up to:
– 23.5 Mbps for data traffic (asynchronous access)
– 2 Mbps for real-time traffic
• H/2@5 GHz provides bit rates up to 54 Mb/s (as
IEEE 802.11a)
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General Characteristics
• Stationary or slowly moving nodes (speed up to
36 Kmph)
• Nodes transmission range up to:
– 50 m @ high bit rate
– 800 m @ low bit rate
• Modulation scheme:
– GMSK for H/1
– OFDM for H/2
• Configuration mode: ad hoc or with AP
• Our focus on configuration with AP
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H/2 Protocol Stack
Higher Layers
Link Layer
Convergence Layer
RLC
(control plane)
DLC
(user plane)
MAC
PHY
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H/2 MAC
•
•
•
•
More than one frequency channel available
Over each channel, TDD/TDMA access scheme
Time is slotted - Frame duration=2 ms
Dynamic capacity assignment in uplink and downlink
MAC-Frame
BCH
FCH
ACH
MAC-Frame
MAC-Frame
DL phase
UL phase
RCHs
Broadcast CH - Frame CH- Access feedback CH - Random CH
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H/2 MAC: Transport Channels
• Broadcast CHannel (BCH): In DL to convey
information concerning the whole radio cell, e.g.,
AP ID, network ID, etc.
• Frame CHannel (FCH): In DL to convey
information on the MAC frame structure (e.g.,
resource grant announcement)
• Access feedback CHannel (ACH): In DL to
transport ack or nack to transmission requests
sent by the terminals in previous frame
• Random CHannel (RCH): In UL to send signaling
data (e.g., resource request, association request)
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H/2 MAC
• A resource request to the AP contains the number of
PDUs that are waiting to be transmitted
• Requests sent using ALOHA scheme, in the
correspondance of the time slots allocated by AP
• Number of contention slots determined by AP
depending on required max/mean delay access
• In case there is not a collision, a node is notified by
AP through ACH in the next frame
• In case of collision, the node computes a backoff time
as a random number of time slots
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H/2 MAC
• If resource request is successfully, the node
passes to contention-free mode
• In contention-free mode, AP schedules
uplink/downlink transmissions
• Periodically, AP can ask nodes about their buffer
occupancy level
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H/2: RLC
• Authentication and other security functions
• RRC, handover management, power saving and
power control
• Establishment and release of user connections
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H/2 DLC - Error Control
• Acknowledged mode: ARQ scheme (SR-like)
• Repetition mode: repetion of the transfered data
without using any feedback channel
• Transmission of some PDUs is repeated
(retransmitted PDUs arbitrary chosen by the sender)
• Receiver accepts all PDUs having a sequence
number within the receiver window
• Unacknowledged mode: use PDU sequence
numbers. PDUs in error are discarded while correct
PDU are passed to higher layers
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H/2 Convergence Layer
• Mapping between higher layer connections /
priorities and DLC connections / priorities
• Flexible amount of QoS classes
• Segmentation and reassembly to / from 48-byte
packets
• Multicast & broadcast handling
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Hiperlan vs. 802.11
•
Similarities:
1. Support ad hoc and with AP configuration
2. Use OFDM
3. Contention-based channel access
4. Bit rate comparable to wired LAN
5. LLC same as in wired LAN
•
Differences:
1. TDD/TDMA in Hiperlan, CSMA/CA in 802.11
2. In Hiperlan more attention to real-time traffic
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